AISC LRFD 1.pdf

180 LOCAL BUCKLING [Comm. B5.TABLE C-B5.1Limiting Width-Thickness Ratios for CompressionElementsDescription ofElementFlanges of I-shaped sections(including hybrid sections) andchannels in flexure [a]Webs in combined flexural andaxial compressionWidth-ThicknessRatiob / tLimiting Width-thickness Ratios pNon-seismich / For P / φ P ≤ 0.125t wu b yFor P / φ P >0.125u b ySeismic0.38 E/Fy0.31 E/FyE æ 2.75Pöu3.76 ç 1-÷Fy è fbPyøE æ 1.54Pöu3.05 ç 1-÷Fy è fbPyøE æ P öuE1.12 ç 2.33- ÷ ³ 1.49Fy è fbPy ø Fy[a] For hybrid beams use F yf in place of F yA greater inelastic rotation capacity than provided by the limiting values p given inTable C-B5.1 may be required for some structures in areas of high seismicity. It hasbeen suggested that in order to develop a ductility of from 3 to 5 in a structural member,ductility factors for elements would have to lie in the range of 5 to 15. Thus, inthis case it is prudent to provide for an inelastic rotation of 7 to 9 times the elasticrotation (Chopra and Newmark, 1980). In order to provide for this rotation capacity,the limits p for local flange and web buckling would be as shown in Table C-B5.1(Galambos, 1976).More information on seismic design is contained in the AISC Seismic Provisionsfor Structural Steel Buildings (AISC, 1997) and the Seismic Provisions for StructuralSteel Buildings Supplement No. 1 (AISC, 1999).Another limiting width-thickness ratio is r , representing the distinction betweennoncompact sections and sections with slender compression elements. As long asthe width-thickness ratio of a compression element does not exceed the limitingvalue r , local elastic buckling will not govern its strength. However, for those caseswhere the width-thickness ratios exceed r , elastic buckling strength must be considered.A design procedure for such slender-element compression sections, basedon elastic buckling of plates, is given in Appendix B5.3. The effective width EquationA-B5-12 applies strictly to stiffened elements under uniform compression. Itdoes not apply to cases where the compression element is under stress gradient. Amethod of dealing with the stress gradient in a compression element is provided inSection B2 of the AISI Specification for the Design of Cold-Formed Steel StructuralMembers (1996). Exceptions are girders with slender webs. Such plate girdersare capable of developing postbuckling strength in excess of the elastic bucklingLRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION